<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
</head>
<body>
<div id="app">
  <h1>{{message}}</h1>
</div>

<script src="../js/vue.js"></script>
<script>
  //let(变量)和const(常量)
  const app = new Vue({
    el: '#app',
    data: {
      message: '中国牛蛙！'
    }
  });
  app.$data.message = '华夏牛蛙！';

  //不允许
  // app = new Vue({
  // })

  //不允许
  // const hh;
</script>
</body>
</html>